1) Field of the Invention
This invention is for blocking and diffusing sunlight outside of a window or skylight or door or other surface that would benefit from reduced infrared heating (especially from but not limited to sunlight). The sunlight control and resulting energy savings occur while a view through the window is maintained. Although the invention may commonly be said to create a shading effect, the invention is not merely providing SHADE. Shade is the blockage of sunlight and the shading object can be quite a distance from the target surface. (The moon shades during an eclipse of the sun). As desirable as dense shade is for IR blockage, because of its shape and distance to the target, the shade may only last for a short time as the sun progresses across the sky. A grate near a window, however, produces a different mix of visible light and can be more efficient at reducing IR heating over the course of the daylight hours.
For a window application, a grate can be mounted to the building or to the window frame, or to the sash. It can also be deployed as a shutter or shutter insert. It can be deployed as an awning or awning insert. For a skylight it can additionally be laid on the windowpane and constrained so as to stay on the frame assembly. For East or West facing windows or where a functional angle is difficult to achieve with standard mounting near parallel to the windowpane, a slant type grate can be used to more effectively block direct sunlight while admitting diffused visible light. Alternatively, for East or West facing windows the cell depth can be increased both to achieve a functional angle for more time during the day and to increase IR absorption. Grates are typically installed during warm months when the air conditioner is active and stored in cooler months when the air conditioner is dormant.
Because an installed grate changes the appearance of a window from outside the building, a thin (such as 0.06 to 0.09″) sheet of acrylic can be installed on the outside of the grate. This restores a reflective surface to the window.
For all embodiments, ultraviolet light (UV) is also blocked, absorbed, or diffused by the grate and this may reduce problems with fading of materials inside the building but this is not the focus of the invention.
2) Statement of Problems and Related Art
Occupants of a building or structure have multiple desires from windows. This includes a good view (from either side), low initial cost, minimal operating adjustments required, favorable impact on heating and cooling costs, good appearance, and the ability to admit desirable sunlight wavelengths while blocking undesirable wavelengths. For well insulated and sealed houses, the largest winter heat loss per area is through the many windows and the largest summer daytime heating is through those same windows. That summer heat in turn creates peak demands for air conditioning and for the Utility. The following methods are options for enhancing window performance:
a) Internal curtains and blinds. These are part of the interior decor and control visible light while providing privacy. Blinds can reflect some IR back outside when the blinds are closed and the view is obscured, but this requires occasional adjustment and produces only a slight reduction in cooling costs. An improvement to blinds is the collapsible shade documented in U.S. Pat. No. 5,701,939. The shade blocks direct sunlight from impinging other interior surfaces.
b. Awnings. These are external sunshades and the structure is sometimes collapsible. The awning typically has a solid sheet or mesh screen as a covering and working surface. Awnings are mounted a distance from the windowpane to allow a view around the awning. If the awning is large enough to fully shade a window for all day it would detract from the view, appearance, and initial cost objectives. Awnings are best retracted or removed and stored out of season.
c) Screens. Of interest here are sunscreens. Insect screens provide little shade. Although a screen can be a perforated sheet it is usually a thin woven mesh of metal or fiber. The mesh can be installed on a fixed or variable frame or on no frame at all for rollup versions. Screens have low initial cost for the mesh itself and when mounted externally, provide shade in inverse proportion to the open area of the weave. The view through a screen is partially obscured so there's a tradeoff Open meshes are easier to see through but block less IR and other wavelengths.
d) Shutters. When installed outside the building, a shutter blocks summer sunlight. A common design involves louvers and the view is limited. Internal shutters exist, often with adjustable louvers, but inside shutters allow the IR inside and only mildly reduce cooling costs.
e) Architectural overhangs. These function like awnings and are helpful in blocking direct sunlight in summer while passing it in winter. Overhangs are permanent structures and add to the cost of the building. They are rarely added after initial construction.
f) Film applied to the windowpane. Some films are more selective to IR blockage than others. If a film blocks IR in warm weather months but remains in place all year it detracts from desirable winter heating by sunlight. If so, the economic benefits in summer are eroded. Films commonly have some darkening effect on the interior and some of the visible light is rejected.
g) Blinds between the double panes. This design is effective in blocking some IR before it reaches the room interior. The blinds can be adjusted manually to provide variable light attenuation, a variable appearance, and a variable view. IR absorbed by the blinds heats the median space between the panes. Additionally, because the blinds are a collection of horizontal slats they allow some sunlight to penetrate from the sides.
h) Diffraction grates. Light entering perpendicular to the plane of the grate has visible wavelengths passed while reducing IR passed. As the seasons change and the sunlight approaches from a more shallow angle, both are favorably passed (U.S. Pat. No. 6,014,845 Jain). The application is especially useful for skylights and the diffraction grate can be removed for winter.
i) Other methods: Chromogenic coatings, shade trees, free standing solar filters of perforated sheet, low transmittance glass, and solid shutters.
Functional Angle Defined. The functional angle is used to orient the grate with respect to the window and the position of the sun to optimize the performance and the mounting. The easiest way to determine a functional angle is to hold the grate in the sunlight while observing the shadow cast. If the grate plane is perpendicular to the sun only a faint shadow will result. As the grate is tilted and rotated, the observer will note a definite grid or waffle pattern of light and shadow. Further manipulation will cause the light portion of the pattern to diminish. When the pattern is at the transition point from a mix of light and shadow to all shadow, this establishes a functional angle for that place and time. Because of the three dimensional nature of the grate, there will be multiple solutions for a functional angle and this is beneficial once the grate is installed in a fixed position near a surface to be protected from IR heating.
Open Area Defined. In the plane of the grate, the open area is that portion not occupied by the structure of the grate. The percent open area is: Open area/Total area. A solid grate with cell structure filled would have no open area.